1. Field of the Invention
This invention relates to a pneumatic tire comprising a carcass formed by putting cords to an outside of a support and a method of building the same.
2. Description of Related Art
As the pneumatic tire comprising a carcass formed by putting cords to an outside of a support, there has hitherto been known a tire described, for example, in JP-A-6-171306. In such a tire, the carcass is comprised of a cord toroidally extending between a pair of bead portions in a meridional direction, which consists of many arch portions arranged over a full circumference of the tire so as to separate away from each other at a constant pitch C in a center of the tread in the circumferential direction, and many circumferential portions alternately connecting radially inward ends of the adjacent arch portions located in both bead portions to each other and extending substantially in the circumferential direction. In order to anchor the cord of the carcass in the bead portion, inner and outer bead layers are formed by spirally winding a bead cord plural times at the outside and inside of the circumferential portion in the axial direction of the tire in the vicinity of the radially inward end of the arch portion, respectively, A a high-hardness rubber layer is arranged between the inner bead layer and the carcass and between the outer bead layer and the carcass to join the inner and outer bead layers to each other, whereby it is prevented to get out the circumferential portion of the carcass outward in the radial direction of the tire.
In the conventional pneumatic tire, however, there is a problem that since each of the circumferential portions is made from a single fine cord, even if the high-hardness rubber layer is arranged between the carcass and the inner bead layer and between the carcass and the outer bead layer as mentioned above, the getting out of the circumferential portion can not sufficiently be prevented. Also, the distance in the circumferential direction between the arch portions constituting the carcass or a constant pitch C is narrow, so that it is difficult to conduct the putting work of the cord.
It is, therefore, an object of the invention to provide a pneumatic tire being simple in the building and capable of strongly anchoring cords constituting the carcass in the bead portion as well as a method of building the same.
According to a first aspect of the invention, there is the provision of a pneumatic tire comprising a carcass made from at least one cord array comprised of many arch portions of a cord extending between a pair of bead portions in a meridional direction and arranged over a full circumference of the tire at an equal interval P in a circumferential direction of the tire and many circumferential portions of the cord alternately arranged in the bead portions, each connecting ends of adjacent arch portions to each other in the bead portion and extending substantially in the circumferential direction, provided that the cord array is arranged at n layers (n is an integer of 2 or more) while offsetting these layers by a distance L obtained by dividing the interval P by n in the circumferential direction and the circumferential portions of these layers made from the cord array are substantially contacted with each other to from an overlap region, and a bead wire reinforcing layer formed by winding a bead wire plural times in the circumferential direction and arranged so as to sandwich the cord array in the vicinity of a boundary between the arch portion and the circumferential portion from both sides thereof to thereby strongly anchor the overlap region of the carcass by the bead wire reinforcing layer.
According to a second aspect of the invention, there is the provision of a method of building a pneumatic tire, which comprises the steps of:
arranging an inner bead wire reinforcing layer in a position corresponding to a bead portion by winding a bead wire on an outside of a support attached with an inner rubber layer plural times in a circumferential direction of the support;
shaping a cord array comprised of many arch portions and many circumferential portions by repeating over a full circumference of the support a working operation that a cord is extended from one of the bead portions to the other bead portion along the outer surface of the support in a meridional direction to form a toroidal arch portion and extended substantially in the circumferential direction at the other bead portion by a distance P to form a circumferential portion and again extended from the other bead portion to the one bead portion in the meridional direction to form another arch portion and again extended substantially in the circumferential direction in the one bead portion by the distance P to form another circumferential portion;
repeating the shaping of the cord array n times (n is an integer of 2 or more) while offsetting by a distance L obtained by dividing the distance P by n in the circumferential direction and substantially contacting the circumferential portions in each of the bead portions with each other to form a carcass;
arranging an outer bead wire reinforcing layer by winding a bead wire plural times in the circumferential direction at the outside of the support to sandwich the cord array in the vicinity of a boundary between the arch portion and the circumferential portion from both sides thereof together with the inner bead wire reinforcing layer in each bead portion to thereby strongly anchor an overlap region formed by the contact of the circumferential portions by the inner and outer bead wire reinforcing layers.
In the invention, the carcass is formed by arranging the cord array comprised of many arch portions separated from each other at an equal distance P in the circumferential direction and many circumferential portions alternately arranged in the bead portions and connecting the ends of the adjacent arch portions to each other at n layers (n is an integer of 2 or more) by the distance L, so that the distance P between the arch portions constituting the cord array is wider by n times than the constant pitch C (equal to the above distance L) in the conventional technique and hence the putting work of the cord for the formation of the carcass is simple and easy. And also, the carcass has an overlap region formed by substantially contacting the circumferential portions with each other, so that these circumferential portions are restrained with each other by the contact in the overlap region and hence the size of the circumferential portion as a whole is apparently larger than that of the circumferential portion in the single cord. Furthermore, the bead wire reinforcing layers are arranged so as to sandwich the cord array from both sides thereof in the vicinity of a boundary between the arch portion and the circumferential portion located outward from the overlap region in the radial direction, so that the overlap region is strongly anchored by these bead wire reinforcing layers to surely prevent the getting out from the bead portion.